Answer
The cost of taking a ski trip instead of working includes not only the actual price of the ski trip but also the wages that you give up by not working.
The cost of taking a ski trip instead of studying includes not only the actual price of the ski trip but also the reduced grade that you may get as a result of not studying.
Work Step by Step
The cost of a good or service includes not only the actual price, but any other benefits that you have to give up in order to purchase the good or service.